1. Field of the Invention
The present invention relates to a surface analysis and measurement method based on the flow resistance of a fluid, and an atomic force microscope using the method. More specifically, the present invention relates to a method for precisely analyzing and measuring the surface of a nanometer-sized sample by utilizing flow resistance resulting from a flow of a fluid, and an atomic force microscope using the method.
2. Description of the Related Art
SPM is an abbreviation for scanning probe microscope and is a generic term for a new concept of microscope capable of measuring the surface characteristics of materials at the atomic level. In Korea, SPM is simply called an atomic microscope. SPM has overcome the generally accepted truth that atoms are too small (0.1-0.5 nm) to be seen even with the aid of very advanced microscopes. Optical microscopes, scanning electron microscopes (SEMs) and scanning probe microscopes are considered as belonging to the first, second and third generation microscopes, respectively. Optical microscopes have a maximum magnification of thousands of times and the scanning electron microscopes have a maximum magnification of tens of thousands of times, while scanning probe microscopes have a maximum magnification of tens of millions of times, which is sufficient to observe individual atoms. Transmission electron microscopes (TEMs) have a horizontal resolution on the atomic scale, but they have too low a vertical resolution to observe individual atoms. In contrast, scanning probe microscopes have a higher vertical resolution than their horizontal resolution, thus enabling the observation of samples smaller than 0.01 nm corresponding to one tenths of the diameter of atoms.
FIG. 1 illustrates a system of a prior art atomic force microscope based on the SPM principle.
The atomic force microscope (AFM) is an instrument for measuring the surface topography of a sample by using a tiny bar, called a cantilever. The cantilever has a structure in which a cantilever probe having a size as small as a few nm is formed on one end of a cantilever body.
When the probe approaches the surface of the sample, attractive and repulsive forces act between atoms of the probe tip and atoms of the sample surface depending on the distances between the probe tip and the sample surface. The degree of bending of the cantilever by the forces represents the surface topography of the sample. This is the basic principle of the atomic force microscope.
Atomic force microscopes are divided into two modes, i.e. contact mode and non-contact mode, depending on whether they utilize a repulsive force or an attractive force. Particularly, the non-contact mode atomic force microscope measures the topography of a sample by using a force gradient rather than by using a direct force in a state in which a probe is relatively spaced away from the sample. Accordingly, the actual force applied to the sample in the non-contact mode atomic force microscope is much smaller than the repulsive force in the contact mode atomic force microscope, thus enabling the measurement of soft samples susceptible to damage. The topology measurement of a sample using atomic forces is done by reading coordinates of moments when changes of a cantilever including a probe due to the atomic forces can be detected as the cantilever approaches the sample.
A scanning probe microscope is an instrument by which various characteristics (e.g., surface topography) of a sample can be measured and analyzed by moving a tiny probe close to the sample to induce interactions between the sample and the probe tip.
Scanning probe microscopes are currently realized in various forms depending on their measurement principles.
An atomic force microscope (AFM), known as the most common scanning probe microscope, includes a small-sized bar (10 μm×1 μm), called a cantilever, and a probe disposed at a distal end of the cantilever. These elements are made by micro-machining.
The probe is made by the following procedure. First, a thin tungsten filament is electrochemically etched to make a tip sharp. Only a few atoms only are left on the distal end of the tip. This sharp needle is made very sensitive in the presence of a strong electric field at a high temperature. Finally, an oxide layer formed during etching is removed from the tungsten filament.
When an appropriate voltage is applied between the probe tip and a conductor sample in a state in which the probe is spaced a distance of 0.5 nm, which corresponds to the size of one or two atoms, from the sample surface, electrons penetrate and pass through the energy battier between the probe and the sample. As a result, a current flows between the probe and the sample (“quantum mechanical tunneling”). This phenomenon takes place because the two conductors are very close to each other. A long distance between the probe and the sample drastically decreases the possibility of electron tunneling, leading to a marked reduction in the amount of current.
The probe is moved up, down, right and left by the action of a scanner made of a piezoelectric ceramic. The scanner has a precision of at least 0.01 nm.
The probe scans right, left, forward and backward along the sample surface while adjusting its height so as to allow the current to flow at a constant rate. At this time, the upward and downward movement distances of the probe at given points are recorded and represented as brightness values on a computer screen. This image represents the topography of the sample. Data from the image include flatness, a sectional diagram and a three-dimensional diagram of each portion of the sample and other statistical data, as well as a top diagram of the sample.
However, a problem of the prior art atomic force microscope is that the probe mechanically moves along the sample surface, which renders the imaging speed slower than that of an optical microscope or an electron microscope.
Further, a flexible size of the probe makes it difficult or impossible to measure steeply inclined surface portions like narrow and deep valleys.
Moreover, convolution effect due to the shape of the probe causes measurement errors even in measurable portions of the sample. Furthermore, the atomic force microscope suffers from poor accuracy of operation.